* Remove `Plausible.Sites.set_allowed_event_props/2` function

This commit removes the `Plausible.Sites.set_allowed_event_props/2`
function in favor of `Plausible.Props.allow/2`.

* Allow internal prop keys in queries

This commit allows internal prop keys by default in queries. I decided
not to include internal keys in the database, because they might change
and we'd need to migrate it again.

* remove redundant 'props.' from behaviours/index.js

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Co-authored-by: Robert Joonas <robertjoonas16@gmail.com>